wow you think CRE is meh??? CRE is ridiculously powerful if you share borders with a few civs. I admit it's a lot less useful if you find yourself isolated or squeezed into a peninsula or something, otherwise it's amazing. I think SPI and PRO are tied for the worst trait, and NO WHERE near PHI/EXP. Then again, I don't play Civ spreadsheet style. I guess if you're still playing the retail version of BTS on deity, SPI might be useful, as you said, for diplomacy. I play Kmod, random personalities, so I don't know what any AI is going to do.
It depends on how you settle really, which is map dependent and difficulty dependent. On Deity where you space your cities close to your capitol because maintenance costs are ridiculous, having your full-cross is very likely. When you set-up far flung blocker cities, having your full-cross is less guaranteed so CRE has an additional + to it. For the games I play, CRE essentially means -30h per city and fast Libraries, being essentially assured my full-cross early game, doesn't enter the equation.
I haven't played K-Mod and largely from what I understand exploiting it's AI is significantly harder (Random personalities doesn't really make a huge impact, it's pretty easy to figure out what leader has what AI) so SPI loses the vast majority of it's power. If diplomatic shenanigans don't enter the equation, SPI saves you some hammers and anarchy turns, hardly even close to as strong as PHI/EXP, if diplomacy is exploitable, it's on par, just because of how powerful diplomacy is.